The present invention relates to an antenna structure used in a portable radio device and, more particularly, to an antenna structure suitable for a portable radio device operating in a UHF (ultra high frequency) band.
A loop antenna has been employed in a portable radio device, such as a paging receiver, for its compactness. The loop antenna has strong directivity. Thus, if the radio device is held so that the loop antenna is in the direction of minimum sensitivity, the signal reception sensitivity will be deteriorated. This problem will be discussed herein referring to the accompanying drawings.
In order to avoid the problem mentioned above, it is known to use a loop antenna with a slender conductive element which is placed along a line perpendicular to a plane including the loop antenna and intersecting at a center of the loop antenna and which is inductively coupled to the loop antenna. This antenna structure is, however, not suitable for a portable radio device due to its bulkiness.